The quaternion group as a subgroup of the sphere braid groups Daciberg Gonçalves, John Guaschi To cite this version: Daciberg Gonçalves, John Guaschi. The quaternion group as a subgroup of the sphere braid groups. Bulletin of the London Mathematical Society / The Bulletin of the London Mathematical Society, 2007, 39 (2), pp.232-234. 10.1112/blms/bdl041. hal-00020804 HAL Id: hal-00020804 https://hal.archives-ouvertes.fr/hal-00020804 Submitted on 15 Mar 2006 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. The quaternion group as a subgroup of the sphere braid groups DACIBERG LIMA GONC¸ALVES Departamento de Matem´atica - IME-USP, Caixa Postal 66281 - Ag. Cidade de S˜ao Paulo, CEP: 05311-970 - S˜ao Paulo - SP - Brazil. e-mail: [email protected] JOHN GUASCHI Laboratoire de Math´ematiques Emile Picard, UMR CNRS 5580, UFR-MIG, Universit´eToulouse III, 31062 Toulouse Cedex 9, France. e-mail: [email protected] 15th March 2006 Abstract Let n ≥ 3. We prove that the quaternion group of order 8 is realised as a subgroup of 2 the sphere braid group Bn(S ) if and only if n is even. If n is divisible by 4 then the 2 commutator subgroup of Bn(S ) contains such a subgroup. Further, for all n ≥ 3, 2 Bn(S ) contains a subgroup isomorphic to the dicyclic group of order 4n. The braid groups Bn of the plane were introduced by E. Artin in 1925 [A1, A2], and were generalised by Fox to braid groups of arbitrary topological spaces using the notion of configuration space [FoN]. Van Buskirk showed that the braid groups of a compact connected surface M possess torsion elements if and only if M is the sphere S2 or the real projective plane RP 2 [VB]. Let us recall briefly some of the properties of the braid groups of the sphere [FVB, GVB, VB]. 2 2 2 If D ⊆ S is a topological disc, there is a group homomorphism ι: Bn → Bn(S ) induced by the inclusion. If β ∈ Bn then we shall denote its image ι(β) simply by β. Then 2 Bn(S ) is generated by σ1,...,σn−1 which are subject to the following relations: σiσj = σjσi if |i − j| ≥ 2 and 1 ≤ i, j ≤ n − 1 ccsd-00020804, version 1 - 15 Mar 2006 σiσi+1σi = σi+1σiσi+1 for all 1 ≤ i ≤ n − 2, and 2 σ1 ··· σn−2σn−1σn−2 ··· σ1 = 1. 2 Consequently, Bn(S ) is a quotient of Bn. The first three sphere braid groups are finite: 2 2 2 B1(S ) is trivial, B2(S ) is cyclic of order 2, and B3(S ) is a ZS-metacyclic group (a group 2000 AMS Subject Classification: 20F36 (primary) 1 whose Sylow subgroups, commutator subgroup and commutator quotient group are all 2 cyclic) of order 12. The Abelianisation of Bn(S ) is isomorphic to the cyclic group Z2(n−1). 2 The kernel of the associated projection ξ : Bn(S ) → Z2(n−1) (which is defined by ξ(σi)= 1 2 2 for all 1 ≤ i ≤ n − 1) is the commutator subgroup Γ2 (Bn(S )). If w ∈ Bn(S ) then ξ(w) is the exponent sum (relative to the σi) of w modulo 2(n − 1). The torsion elements of the braid groups of S2 and RP 2 were classified by Murasugi [M]: 2 if M = S and n ≥ 3, they are all conjugates of powers of the three elements α0 = 2 σ1 ··· σn−2σn−1 (which is of order 2n), α1 = σ1 ··· σn−2σn−1 (of order 2(n − 1)) and α2 = 2 th th th σ1 ··· σn−3σn−2 (of order 2(n − 2)) which are respectively n ,(n − 1) and (n − 2) roots 2 n of Tn, where Tn is the so-called ‘full twist’ of Bn(S ), defined by Tn = (σ1 ··· σn−1) . If 2 2 n ≥ 3, Tn is the unique element of Bn(S ) of order 2 and generates the centre of Bn(S ). 2 In [GG2], we showed that Bn(S ) is generated by α0 and α1. 2 For n ≥ 4, Bn(S ) is infinite. It is an interesting question as to which finite groups are 2 realised as subgroups of Bn(S ) (apart of course from the cyclic groups hαii). In [GG2], 2 2 we proved that Bn(S ) contains an isomorphic copy of the finite group B3(S ) of order 12 if and only if n 6≡ 1 mod 3. The quaternion group Q8 of order 8 appears in the study of braid groups of non-orientable surfaces, being isomorphic to the 2-string pure braid group 2 2 2 P2(RP ). Further, since the projection F3(RP ) → F2(RP ) of configuration spaces of RP 2 onto the first two coordinates admits a section [VB], it follows using the Fadell- 2 Neuwirth short exact sequence that P3(RP ) is a semi-direct product of a free group of rank 2 by Q8 [GG1]. While studying the lower central and derived series of the sphere braid groups, we 2 showed that Γ2 (B4(S )) is isomorphic to a semi-direct product of Q8 by a free group of rank 2 [GG3]. After having proved this result, we noticed that the question of the 2 realisation of Q8 as a subgroup of Bn(S ) was explicitly posed by R. Brown in connection with the fact that the fundamental group of SO(3) is isomorphic to Z2 [ATD]. In this paper, we give a complete answer to this question: Theorem. Let n ∈ N, n ≥ 3. 2 (a) If n is a multiple of 4 then Γ2 (Bn(S )) contains a subgroup isomorphic to Q8. 2 (b) If n is an odd multiple of 2 then Bn(S ) contains a subgroup isomorphic to Q8. 2 (c) If n is odd then Bn(S ) contains no subgroup isomorphic to Q8. Proof. We first suppose that n is even, so that n = 2m with m ∈ N. Let H be the 2 subgroup of B2m(S ) generated by x and y, where x =(σ1 ··· σ2m−1)(σ1 ··· σ2m−2) ··· (σ1σ2)σ1, −1 −1 −1 y =(σ1 ··· σm−1)(σ1 ··· σm−2) ··· (σ1σ2)σ1 · σ2m−1(σ2m−2σ2m−1) ··· −1 −1 −1 −1 ··· (σm+2 ··· σ2m−1)(σm+1 ··· σ2m−1). Geometrically, x may be interpreted as the ‘half twist’ or Garside element of B2m [Bi]. Further, y may be considered as the commuting product of the positive half twist of the 2 first m strings with the negative half twist of the last m strings. Then x = T2m and 2 m −1 −1 m S2 y = (σ1 ...σm−1) (σm+1 ...σ2m−1) = T2m in B2m( ) (cf. [FVB, GVB]). It is well −1 2 known that xσix = σ2m−i in B2m [Bi], and thus in B2m(S ), from which we obtain −1 −1 xyx = y . Hence H is isomorphic to a quotient of Q8. But x is of order 4, and the induced permutation of y on the symmetric group S2m is different from that of the elements of hxi. It follows that H contains the five distinct elements of hxi∪{y}, and 2 ∼ 2 so H = Q8. If m is even then x, y ∈ Ker(ξ), and thus H ⊆ Γ2 (Bn(S )). This proves parts (a) and (b). 2 To prove part (c), suppose that n is odd, and suppose that x, y ∈ Bn(S ) generate a 2 2 −1 −1 subgroup H isomorphic to Q8, so x = y and xyx = y . In particular, x and y are of ±(n−1)/2 order 4, and thus are conjugates of α1 by Murasugi’s classification. By considering ε1(n−1)/2 ε2(n−1)/2 −1 a conjugate of H if necessary, we may suppose that x = α1 and y = wα1 w , 2 −1 where w ∈ Bn(S ) and ε1,ε2 ∈{1, −1}. Replacing x by x if necessary, we may suppose ε1(n−1)/2 further that ε1 = −ε2. Thus xy = hα1 ,wi, and is of exponent sum zero. On the ±(n−1)/2 other hand, xy is an element of H of order 4, and so is conjugate to α1 by Murasugi’s ± − − ξ α (n 1)/2 n(n 1) n classification. But 1 = ± 2 , which is non zero modulo 2( −1). This yields a contradiction, and proves part (c). Remark. Let n ≥ 3. Using techniques similar to those of the proof of the Theorem, 2 one may show that the subgroup of Bn(S ) generated by σ1 ··· σn−1 and the half twist x is isomorphic to the dicyclic group of order 4n. In particular, if n is a power of two 2 then Bn(S ) contains a subgroup isomorphic to the generalised quaternion group of order 2 2 4n. Further investigation into the finite subgroups of Bn(S ) and Bn(RP ) will appear elsewhere. References [ATD] Algebraic topology discussion list, January 2004, http://www.lehigh.edu/~dmd1/pz119.txt. [A1] E. Artin, Theorie der Z¨opfe, Abh. Math. Sem. Univ. Hamburg 4 (1925), 47–72. [A2] E. Artin, Theory of braids, Ann. Math. 48 (1947), 101–126. [Bi] J. S. Birman, Braids, links and mapping class groups, Ann.